The Third Generation Partnership Project (3GPP) is standardizing a first release of a Long Term Evolution (LTE) concept. An LTE system may include a number of base stations (also referred to as “Node Bs”) connected to one or more mobility management entities/serving gateways (MME/S-GWs). A number of nodes (e.g., a network management system (NMS) node, an operation and support system (OSS) node, etc.) may provide operation and maintenance functionality for the base stations and/or the MMEs/S-GWs). In LTE, a downlink is based on orthogonal frequency division multiplexing (OFDM), while an uplink is based on a single carrier modulation method known as discrete Fourier transform spread OFDM (DFT-S-OFDM).
During initial access, user equipment (UE) seeks access to a network (e.g., a radio network) in order to register and commence services. A LTE random access (RA) procedure serves as an uplink control procedure to enable the UE to access the network. Since the initial access attempt cannot be scheduled by the network, the RA procedure is contention based. Collisions may occur and an appropriate contention-resolution scheme needs to be implemented. Including user data on the contention-based uplink is not spectrally efficient due to the need for guard periods and retransmissions. Therefore, transmission of a RA burst (e.g., a preamble) is separated. The RA burst obtains uplink synchronization from the transmission of user data.
The LTE RA procedure permits the UE to align its uplink timing to timing expected by a base station in order to minimize interference with other UE transmissions. Uplink time alignment is a requirement in an evolved universal terrestrial radio access network (E-UTRAN) before data transmissions can commence. The LTE RA procedure also provides a mechanism for the UE to notify the network of its presence and enables the base station to provide the UE initial access to the network. The LTE RA procedure is also used when the UE has lost uplink synchronization or when the UE is in an idle or a low-power mode.
The basic LTE RA procedure is a four-phase procedure. A first phase (e.g., phase 1) includes transmission of a random access preamble, which permits the base station to estimate the transmission timing of the UE. Uplink synchronization is necessary in the first phase since the UE otherwise cannot transmit uplink data. A second phase (e.g., phase 2) includes the network transmitting a timing advance command to correct uplink timing based on timing of an arrival measurement in the first phase. The second phase also assigns uplink resources and a temporary identifier to the UE to be used in a third phase of the LTE RA procedure. The third phase (e.g., phase 3) includes signaling from the UE to the network using an uplink shared channel (UL-SCH) similar to normally scheduled data. This UL-SCH signaling uniquely identifies the UE. The exact content of the UL-SCH signaling depends on the state of the UE (e.g., whether or not it is previously known to the network). A fourth phase (e.g., phase 4) is responsible for contention resolution in case multiple UEs try to access the network on the same resource. For cases where the network knows, in advance, that a particular UE will perform the LTE RA procedure to acquire uplink synchronization, a contention-free variety of the LTE RA procedure may be used that makes it possible to skip the contention resolution process of phases 3 and 4 (e.g., for cases such as arrival to a target cell at handover and arrival of downlink data). Each phase of the LTE RA procedure is explained in more detail below.
In the first phase of the LTE RA procedure, prior to sending a preamble, the UE synchronizes to the downlink transmissions and reads a broadcast channel (BCCH). The BCCH reveals where the RA time slots are located, which frequency bands can be used, and which preambles (e.g., sequences) are available. At the next RA slot, the UE sends the preamble, where the preamble sequence implicitly includes a random identification (ID) that identifies the UE. For each cell, LTE provides sixty-four such random IDs and thus sixty-four preambles. If multiple RA frequency bands have been defined, the UE randomly selects one of them. The group of sequences allocated to a cell is partitioned into two subgroups. By selecting a preamble sequence from a specific subgroup, the UE can give a single-bit indication of its resource requirement and/or link quality. The particular sequence used for the preamble is randomly selected within the desired subgroup. This sequence implicitly contains a random ID which serves as a UE identifier. The base station estimates the uplink timing of the UE based on the timing of the received preamble.
In the second phase of the LTE RA procedure, after the preamble transmission, the UE waits for a RA response message on a downlink shared channel (DL-SCH) and a downlink assignment which is indicated on a dedicated physical control channel (DPCCH). The RA response message is transmitted semi-synchronously (e.g., within a time window) with the reception of the RA preamble in order to allow a scheduler more flexibility. The RA response message includes the same random UE identity as present in the preamble, a time alignment message to provide a proper uplink timing to the UE, a radio network temporary identifier (RNTI) that is unique for the particular RA resource (e.g., time, channel, and preamble) used in phase 1, and an uplink resource grant for transmission on the UL-SCH in phase 3. If no RA response message has been received after a configurable time following the preamble transmission, the UE applies a backoff procedure. A backoff value in the UE (e.g., signaled to the UE in a past RA response message) delays the transmission to a randomly chosen time (e.g., between zero and the backoff value) before attempting random access again. The UE selects new random parameters for the preamble sequence and the non-synchronized RA frequency band. Furthermore, the UE increases a power level of the preamble to obtain a power ramping procedure (e.g., similar to the procedure used in wideband code division multiple access (WCDMA)).
In phase 3 of the LTE RA procedure, the UE provides the network with a unique identifier in the message it transmits on the UL-SCH according to a grant contained in the RA response message. The type of UE identifier depends on an extent the UE is already known in the network. In the case of initial access, the message is a radio resource control (RRC) connection request message. In the case of non-initial access (e.g., when the UE is already RRC connected), the UE identifier is a cell RNTI (C-RNTI) and is signaled by the media access control (MAC) layer.
In the fourth phase of the LTE RA procedure, the base station echoes the UE identity provided by the UE in phase 3. A terminal that finds a match between the identity received in the fourth phase and the identity transmitted as part of the third phase declares the LTE RA procedure successful. This terminal also transmits a hybrid automatic repeat request (HARQ) acknowledgement in the uplink. For non-initial access (e.g., when the UE is RRC connected), the UE identity is reflected on the DPCCH. If the UE has not yet been assigned a C-RNTI, the temporary identity from the second phase is promoted to the C-RNTI, otherwise the UE keeps its already assigned C-RNTI. Terminals that do not find a match between the identity received in phase 4 and the respective identity transmitted as part of phase 3 are considered to have failed the LTE RA procedure and need to restart the LTE RA procedure with phase 1.
For cases where the network knows, in advance, that a particular UE will perform the LTE RA procedure to acquire uplink synchronization, a dedicated preamble is reserved and assigned to the UE under consideration. Dedicated preamble assignment for handover is handled by RRC, whereas preamble assignment for downlink data arrival is handled by the MAC layer. When the UE transmits the dedicated preamble in phase 1, the network knows to which UE this preamble was assigned and can, at the time of detection of this preamble, determine the identity of the UE. Thus, no contention resolution is needed and the delay before data transmission can resume is reduced.
A single RA opportunity includes a time slot and a fixed bandwidth. The RA time slot length (TRA) accommodates the preamble sent by the UE and the required guard period (GP) to take into account the unknown uplink timing. The timing misalignment amounts to “6.7” microseconds per kilometer (μs/km). For a minimum TRA of one millisecond (ms), the preamble length is “800” μs plus a cyclic prefix of about “102.5” μs. A guard time of “97.5” μs suffices for cell radii up to fifteen km. Larger guard periods and a cyclic prefix are needed to accommodate timing uncertainties from cells larger than fifteen km. Such large cells may also require longer preambles to increase the received energy. In order to support RA under various cell conditions, three additional RA preamble formats have been defined that require a TRA of two or even three milliseconds. These larger slots are created when the base station does not schedule traffic in consecutive sub-frames. The extended preambles contain repetitions of the “800” μs long part and/or a longer cyclic prefix.
For time division duplex (TDD) systems, an additional short RA preamble is defined and spans “133” μs. Because of this very short duration, the preamble may not contain a cyclic prefix but a technique called overlap-and-add may be used to enable frequency-domain processing.
According to 3GPP, the bandwidth of a RA opportunity is “1.08” megahertz (MHz). The effective bandwidth utilized by the RA preamble is “1.05” MHz with small spectral guard bands on each side. This is necessary since RA and regular uplink data are separated in the frequency domain but are not completely orthogonal.
For frequency division duplex (FDD) systems, RA opportunities do not occur simultaneously in different frequency bands but are separated in time. This spreads processing load in a RA receiver. 3GPP defines RA configurations based on how often RA opportunities occur. In total, sixteen configurations are defined, ranging from one RA opportunity every twenty milliseconds (e.g., a very low RA load) to one RA opportunity every one millisecond (e.g., very high RA load).
In TDD, not all sub-frames are downlink reducing sub-frames that can be allocated to RA. To provide such sub-frames in TDD configurations for high RA loads, multiple RA opportunities can be scheduled in a single sub-frame. In order to compensate for the rather low frequency diversity obtained within “1.05” MHz, the RA opportunity hops in the frequency domain. The time division multiple access (TDMA)/frequency division multiple access (FDMA) structure of the RA opportunities in FDD includes one “1.08” MHz band allocated to RA at each time whereas several bands are possible in case of TDD. The RA opportunities occur at the band edges of the physical uplink shared channel (PUSCH) directly adjacent to the physical uplink control channel (PUCCH).
A basic RA preamble is prefixed with a cyclic prefix to enable simple frequency domain processing. The length of the RA preamble is in the order of TGP+TDS=97.5 μs+5 μs=102.5 μs, where TGP corresponds to a maximum round trip time and TDS corresponds to a maximum delay spread. The cyclic prefix ensures that a received signal is circular (e.g., after removing the CP in the RA receiver) and thus can be processed by fast Fourier transforms. Therefore, an active RA preamble duration is 1000 μs−2·TGP−TDS=800 μs. A RA subcarrier spacing is 1/800 μs=1250 Hz.
The RA preamble may include extended preamble formats. A first extended preamble format (e.g., format 1) has an extended cyclic prefix and is suited for cell radii up to approximately “100” km. However, since no repetition occurs, this format is only suited for environments with good propagation conditions. A second preamble format (e.g., format 2) contains a repeated main preamble and a cyclic prefix of approximately “200” μs. With a RA opportunity length of two milliseconds, the remaining guard period is also approximately “200” μs. This format supports cell radii of up to approximately “30” km. A third preamble format (e.g., format 3) also contains a repeated main preamble and an extended cyclic prefix. Using a RA opportunity length of three milliseconds, this format supports cell radii of up to approximately “100” km. As opposed to format 1, format 3 contains a repeated preamble and is therefore better suited for environments with bad propagation conditions.
The requirements of a sequence that includes the RA preamble are two-fold: good auto-correlation function (ACF) properties and good cross-correlation function (CCF) properties. A sequence that has ideal periodic ACF and CCF properties is the Zadoff-Chu sequence. The periodic ACF of the Zadoff-Chu sequence is non-zero at time-lag zero and the magnitude of the CCF is equal to the square-root of the sequence length (N). Due to special properties of Zadoff-Chu sequences, a number of sequences is maximized if N is a prime number. This together with the requirement that the effective RA bandwidth (e.g., N·1250 Hz) fit into “1.05” MHz, leads to N=839.
A Zadoff-Chu sequence of length N can be expressed, in the frequency domain, as:
                    X        ZC                  (          u          )                    ⁡              (        k        )              =          ⅇ                        -          jπ                ⁢                                  ⁢        u        ⁢                              k            ·                          (                              k                +                1                            )                                N                      ,where “u” is the index of the Zadoff-Chu sequence within the set of Zadoff-Chu sequences of length N. From one Zadoff-Chu sequence, multiple preamble sequences can be derived by cyclic shifting. Due to the ideal ACF of Zadoff-Chu sequence, multiple and mutually orthogonal sequences can be derived from a single root sequence by cyclic shifting one root sequence multiple times a maximum allowed round trip time plus a delay spread in the time domain. The correlation of such a cyclic shifted sequence and the underlying root sequence has a peak at the cyclic shift. If the received signal has a valid round trip delay (e.g., not larger than a maximum assumed round trip time), the correlation peak occurs at the cyclic shift plus the round trip delay which is still in the correct correlation zone. For small cells (e.g., up to “1.5” km radii), all sixty-four preambles can be derived from a single root sequence and are therefore orthogonal to each other. In larger cells, all of the preambles cannot be derived from a single root sequence and multiple root sequences must be allocated to a cell. Preambles derived from different root sequences are not orthogonal to each other.
One disadvantage of Zadoff-Chu sequences is their behavior at high frequency offsets. A frequency offset creates an additional correlation peak in the time domain. A frequency offset has to be considered high if it becomes substantial relative to the RA sub-carrier spacing of “1250” Hz (e.g., from “400” Hz upwards). The offset of the second correlation peak relative to the main peak depends on the root sequence. An offset smaller than a cyclic shift (TCS) may lead to wrong timing estimates, whereas values larger than TCS increase a false alarm rate. In order to cope with this problem, LTE has a high speed mode that disables certain cyclic shift values and root sequences so that a transmitted preamble and round trip time can uniquely be identified. Additionally a special receiver that combines both correlation peaks is required. For cells larger than approximately “35” km, no set of sixty-four preambles exists that allows unique identification of the transmitted preamble and estimation of propagation delay (i.e., cells larger than “35” km cannot be supported in high speed mode).
The RA preamble sequences are ordered according to a specified table. The table is designed by separating all packet random access channel (PRACH) sequences into two groups based on a quadrature phase-shift keying (QPSK) cubic metric value using a fixed “1.2” decibel (dB) threshold. The sequences with low cubic metric values are more suitable to assign to large cells than the sequences with high cubic metric values. Within each cubic metric-group (e.g., high and low), the sequences are further grouped according to a maximum allowed cyclic shift (Smax) at high speed.
In LTE, power control for RACH is determined as follows:PRACH(N)=min{PMAX, PO—RACH+PL+(N−1)ΔRACH+ΔPreamble},where PRACH is the preamble transmit power, N=1, 2, 3, etc. is a RACH attempt number, PMAX is a maximum UE power, PO—RACH is a four-bit cell specific parameter signaled via BCCH with a granularity of two dB, PL is a path loss estimated by the UE, ΔRACH is a power ramping step signaled via BCCH and represented by two bits (four levels) with a granularity of two dB, and Δpreamble is a preamble-based offset that is zero for a single preamble transmission and is −3 dB for repeated preambles. The UE will increase its transmission power until network access is granted. There is typically an upper bound (e.g., NMAX) on a number of retransmissions and, thus, a number of power increases.
For optimized operation and performance of RACH, RACH related parameters are cell-specifically configured to adapt the radio conditions and interference situations in the individual cells. This may be achieved by careful radio network planning. However, such a procedure is time-consuming and costly. An alternative is to adjust the RACH parameters via self-optimizing algorithms based on observations in the network. With such settings, the RACH parameters match: an intended cell coverage by preamble cyclic shifts, preamble formats (e.g., a length of cyclic prefix and whether the preamble is repeated or not), power control persistence, and the cubic metric of the root sequences; expected UE velocities by using (or not using) high speed mode preambles; an expected RACH load by the number of RACH opportunities; a detection threshold and an expected interference and noise level by the power control parameters (PO RACH); an expected path loss estimation error and interference uncertainty by the power control parameters (ΔRACH); and RACH requirements received from an operator. This means that for optimized performance, the RACH will not provide better coverage and communication performance than requested.
A specific cell associated with a base station (or the base station) can be temporarily unable to provide service to users in a service area. Service may be unavailable because of, for example, malfunctioning equipment, power outage, transport network failure, etc. This is commonly referred to as “cell outage,” “broken cell,” “downed cell,” “faulty cell,” “out of service cell,” “vulnerable cell,” “malfunctioning cell,” “failed cell,” etc. or another corresponding term if the base station, a site, or a network element is out of service. Furthermore, a cell or a base station may be out of service because of a decision from some mechanism (e.g., to prepare the cell or base station for upgrades, when the need for service is low, such as during low traffic times when it is desirable to shut down network elements to save power, etc.). Such network elements may be referred to as sleeping, dormant, latent, inactive, idle, turned off, shut down, terminated, halted, stopped, ceased, paused, suspended, interrupted, etc. The term “out of service” may be used to indicate a network element that is out of service for any reason (e.g., intended or unintended).
When one or several cells are out of service, surrounding cells may, to some extent, act as replacements for out of service cells or base stations. However, there is a considerable risk that UEs (e.g., mobile telephones) fail to attach to these replacement cells since the UEs' RACH parameters are configured too tightly and only provide coverage in an intended coverage area, given the radio conditions, interference variations, and random access load. For example, three base stations A, B, and C may provide cell coverage that matches RACH coverage. When base station B is out of service, then either base station A or base station C may constitute favorable replacement cells based on downlink evaluations and cell search procedures. However, base stations A and/or C may not be able to serve all UEs formerly served by base station B due to insufficient RACH coverage.